Electrode for high-pressure electric discharge lamps



J. GEMSA Sept. 11, 1962 ELECTRODE FOR HIGH-PRESSURE ELECTRIC DISCHARGE LAMPS Filed June 28, 1960 INVENTDR United States This invention relates to electrodes for high-pressure electric discharge lamps comprising a sealed envelope containing an ionizable gaseous or vapor atmosphere. These electrodes are produced by pressing a mixture of tungsten powder and small additions-of thorium oxide to slug form, sintring the slug form to provide a strongly coherent body, swaging and sometimes drawing said'body to rod form having a diameter of, for instance, aboutone or several millimeters. Said rods are divided into portions of suitable length, and one end of each rod portion is tapered. In case of larger electrode bodies required for'high-pressure discharge lamps of high wattages such a thoriumoxide-containing tungsten rod is, usually, inserted in a metallic block or cylinder of, for instance, sintered tungsten or it is enclosed or supported by other metallic parts.

It'has,.however, been found that these Well-known electrodes donotalways satisfy in operation of electric highpressuredischarge lamps. In lamps with short are lengths and relatively large envelope diameter there occurred sometimes acertain unsilence of'the arc and a very quick destruction of the point of the electrode serving as the cathode.

It is, therefore, an object of the present invention to find the cause'for the aforesaid drawbacks and to overcome them.

Another'objectof the-present invention is'the provision of an electrode for a high-pressure electric discharge lamp so designed as to guarantee always silent burning of the discharge arcand have also a sufficient long life.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing an electrode composed of a solid massive rod having a tapered tip portion and consisting of tungsten and a small addition of thorium oxide, said rod comprising interlocking crystals elongated longitudinally throughout said rod, the length of said crystals amounting at least to several millimeters, the width of said crystals amounting at least to several tenths of millimeters, said rod comprising beside said crystals a plurality of particles minute compared with said crystals, consisting of said thorium oxide and 'being distributed uniformly throughout the whole volume of said rod and enclosed tightly within said coarse crystals. There are also provided process steps for manufacturing the electrodes, whichprocess steps include an annealing procedure for the rod.

The present invention represents a surprise because, as is well known, for thorium oxide-containing tungsten cathodes of electronic discharge tubes a micro-crystalline structure is required in the grain boundaries of which the activating material is concentrated. Such a structure only guarantees in electronic discharge tube cathodes for a longer time a stability between the number of activated atoms passing out from the surface per unit of time and those atoms supplied to the surface in the same time.

To apply this knowledge gained in construction of electronic discharge tubes with thorium oxide-containing tungsten electrodes to high-pressure discharge lamp electrodes would not turn out any success. It has been found that in high-pressure discharge lamps the electrode point atent '0 3,054,014 Patented Sept. 11, 1-962 in case of a fine-crystalline structure impoverishes very quick in respect of thorium oxide whereby pipings or pores are formed bringing about some loosening in the tight structure. In consequence of the absence of thorium-oxide in the electrode point and the destructionof its structure the discharge are appends .to other not yet decomposed places of the electrode around its inactive point and shows thereby the above mentioned unsilence.

In case of electrodes according to the present invention, however, any impoverishment in respectof thorium oxide by diffusion of the thorium oxide particles 'in the electrode and evaporation toward outside, is restrained so much that after a longer burning period a loss in thorium oxide may be observed in a very narrow edge zone of the electrode point only. An impoverishment of "the electrodes in respect of thorium oxide is, obviously, restrained by means of a tight enclosure and uniform dis tribution of the particles in the coarse crystals.

For a better understanding of the present invention reference is made to the accompanying drawings in'which electrodes made according to the present invention are compared with such ones not made as described in the invention and showing bad operation features. In this example cathodes of a xenon high-pressure discharge lamp with a short arc length within a dropor ellipsoidshaped envelope are shown the diameter of which latter is relatively large, designed for direct current operation with a power consumption of about 2 kW. The drawings represent schematical views of longitudinal sections through the rod axis of parts of these cathodes 'in different manufacturingand operating conditions. Similar numerals of reference denote like parts in all the figures.

FIG. 1 shows an etched ground section of a thorium oxide-containing tungsten rod in a preliminary state of cathode manufacture in about tenfold enlargement and FIG. 2 shows an etched ground section of a finished cathode according to the present invention in about tenfold enlargement.

FIG. 3 shows a cutout of an etched ground section-of a cathode without any features of the invention after 152 operating hours which cutout is taken from a range somewhat remote from the point in about two hundredfold enlargement.

FIG. 4 shows a cutout in about seventy five-fold enlargement of a non-etched longitudinal groundsection "of a cathode according to the invention after 900 operating hours.

FIG. 5 shows a cutout in about seventy five-fold enlargement of a non-etched longitudinal ground section of a cathode without any features of the invention after 152 operating hours.

FIG. 6 shows a cutout in about two hundredfold enlargement from the cathode shown'in FIG. 4 which cutout is taken from a range somewhat rem'ote'from the point.

FIG. 7 shows a corresponding cutoutfrom the'cathode shown in FIG. 5.

All the tungsten rods shown in the drawings have a thorium-oxide content of about 5% by weight. Thorium oxide-containing tungsten rods may be manufactured in such a manner that some thorium oxide powder or powdered metallic thorium is admixed to the tungsten powder and that this mixture of powders is pressed, sintered, swaged and, if desired, drawn according to well-known methods. In order to obtain a very uniform distribution of the thorium oxide in tungsten it is, however, more suitable to add, also in accordance with Well-known methods, a. thorium compound in dis-solved form already to the tungstic acid, the starting material for the manufacture of tungsten.

The swaged rod represented in FIG. 1 shows clearly perceivable crystals 1 deformed by the swaging procedure and more or less elongated in the direction of the rod axis.

The proportionately short rod is tapered for the appendage of an arc discharge at 2.

FIG. 2 shows the finished cathode according to the invention. The number of reference 3 designates the partly represented cathode body of sintered tungsten into which the thorium oxide-containing tungsten rod 4 is inserted. The latter was annealed for about 15 minutes in a high-temperature furnace provided with a tube in which the tungsten is annealed in a hydrogen-nitrogen stream at 2800" C. Thereby, the swaged structure shown in FIG. 1 changed by means of complete recrystallization into a coarse crystalline structure with interlocking Crystals 5 elongated longitudinally throughout rod 4, the length of crystals 5 amounting at least to several millimeters, the Width of crystals 5 amounting at least to several tenths of millimeters. A plurality of particles 6, minute compared with crystals 5, consisting of said thorium oxide, is distributed uniformly throughout the Whole volume of rod 4 and enclosed tightly within the coarse crystals 5. There may be applied instead of a single annealing also a succession of annealing, weak deformation and repeated annealing for the purpose of producing the required coarse crystalline structure.

FIG. 3 shows the structure of a cathode of a lamp burning with unsilent are. It shows small polygonal crystallites 7 only somewhat elongated in longitudinal direction which meet with plain grain boundaries. A bad operating behavior was observed also with electrodes having other fine crystalline structures in which the fine crystals were elongated in axial direction.

In the course of operation of a cathode according to the invention its point is reduced little by little and after 900 operating hours, that means nearly at the end of the prescribed life, the appendage of the discharge are at 8 in FIG. 4 appears in a flattened condition. The thorium oxide particles 6 are existing nearly up to the surface 8 still in the original quantity and in the uniform distribution. A narrow edge zone 9 only of a thick ness of 50 to 80 microns is, practically, without any thorium oxide while the rod diameter is of about 4.5 mm.

In case of a cathode not made according to this invention as shown in FIG. 5 there is already after a short burning period of 152 hours the point much more flattened and attacked. There may clearly be seen a strong decomposition of the structure in the flattened cathode point and its spongelike, very porous state. The point is still connected very loose with the base part and seems to be completely lifted in the ground plane. It does, practically, not contain any thorium oxide particles. Only in that range of the cathode which is cooler in operation below the lifted point the thorium oxide particles 10 are maintained in the original quantity.

FIG. 6 represents a very much enlarged cutout of a cathode according to the present invention and shows still better than FIG. 2 how the thorium oxide particles 6 penetrate in uniform distribution the entire coarse crystalline structure. There the particles 6 have a round shape. But they may also be elongated in longitudinal direction of the rod.

FIG. 7 shows a cathode without any features of the invention; there the thorium oxide is embedded in the grain boundaries only of the crystallites 7 in a lineor punctiform manner as it may be seen if compared with FIG. 3. In other electrodes of such lamps which burn with an unsilent arc the thorium oxide particles are found not only in the grain boundaries but project also into the fine crystals elongated in axial direction. It is quite clear that the thorium oxide particles 10 in both these cases under the effect of the discharge are escape in a quicker and more violent manner from the metal than in case of an enclosure within the coarse crystals 5. The numerous, interconnected grain boundaries of the crystallites 7 form, so to say, a network-type structure of smallest capillaries leading to the surface whereby a quick impoverishment in respect of thorium oxide is supported on the cathode. Owing to the :fact that cathode temperature in operation is the highest on the point, the thorium oxide particles 10 will vaporize the quickest at this place. As it may be seen from FIG. 5 it follows therefrom a quick thorium oxide loss on the cathode point and the formation of pores which are formed in the soft tungsten, partly heatedup on certain places to the melting point, by means of the vaporizing thorium oxide.

I claim:

1. An electrode for a high-pressure electric discharge lamp composed of a solid massive rod having a tapered tip portion and consisting of tungsten and a small addition of thorium oxide, said rod comprising interlocking crystals elongated longitudinally throughout said rod, the length of said crystals amounting at least to several millimeters, the Width of said crystals amounting at least to several tenths of millimeters, said rod comprising besides said crystals, a plurality of particles minute compared with said crystals, consisting of said thorium oxide and being distributed uniformly throughout the whole Volume of said rod and enclosed tightly Within said coarse crystals.

2. The process of manufacturing electrodes for high pressure electric discharge lamps as claimed in claim 1 comprising pressing a mixture of tungsten powder and small additions of thorium oxide to slug form, sintering the slug to provide a strongly coherent body, swaging said body to rod form thereby producing a crystalline structure within said rod consisting of fine crystals deformed and elongated by the swaging procedure, dividing said rod into portions of suitable length, tapering one end of each rod portion and annealing said rod portions in a reducing atmosphere up to complete recrystallization and formation of coarse crystals.

References Cited in the file of this patent UNITED STATES PATENTS 1,082,933 Collidge Dec. 30, 1913 1,461,118 Hall July 10, 1925 1,569,095 L'aise Ian. 12, 1926 1,723,862 Jacoby Aug. 6, 1929 2,716,713 Noel Aug. 30, 1955 FOREIGN PATENTS 573,141 Great Britain Oct. 8, 1945 

1. AN ELECTRODE FOR A HIGH-PRESSURE ELECTRIC DISCHARGE LAMP COMPOSED OF A SOLID MASSIVE ROD HAVING A TAPERED TIP PORTION AND CONSISTING OF TUNGSTEN AND A SMALL ADDITION OF THORIUM OXIDE, SAID ROD COMPRISING INTERLOCKING CRYSTALS ELONGATED LONGITUDINALLY THORUGHOUT SAID ROD, THE LENGTH OF SAID CRYSTALS AMOUNTING AT LEAST TO SEVERAL MILLIMETERS, THE WIDTH OF SAID CRYSTALS AMOUNTING AT LEAST TO SEVERAL TENTHS OF MILLIMETERS, SAID ROD COMPRISING BESIDES SAID CRYSTALS, A PLURALITY OF PARTICLES MINUTE CIMPARED WITH SAID CRYSTALS, CONSISTING OF SAID THORIUM OXIDE AND BEING DISTRIBUTED UNIFORMLY THROUGHOUT THE WHOLE VOLUME OF SAID ROD AND ENCLOSED TIGHTLY WITHIN SAID COARSE CRYSTALS. 